Refractories, such as bricks, used to line chemical incinerators usually are required to be resistant to high temperatures and to corrosive gases such as fluorine and hydrofluoric acid. Most refractories for these applications contain some type of silica or silica-containing material. Unfortunately, silica is subject to attack by the corrosive gases which cause silica to be leached through the formation of a gaseous silicon tetrafluoride, SiF.sub.4. This problem is especially severe in insulating backup refractories which traditionally contain high levels of either free or reducible silica usually occurring in the form of a bonding clay.
One disadvantage of high alumina, low silica, insulating refractories is inadequate handling strength after dry pressing. Most conventional refractory mixtures with little or no silica, or silica-containing clay, are so weak that the mixtures cannot be dry pressed and fired into commercially usable shapes. The lack of adequate handling strength is particularly severe in relatively porous alumina refractories which have a low silica content.
U.S. Pat. No. 4,495,301 to Sutor discloses one attempt to produce a lightweight insulating alumina refractory having a low silica content. The composition includes the addition of "hollow spherical particles" of alumina. Although this patent describes a lightweight alumina refractory with a low silica content, the composition requires the addition of a calcium aluminate cement to achieve adequate handling strength. In fact, in Table II - continued (col. 3 and 5, lines 1-15) on the Sutor patent, when the calcium aluminate cement wt. % was reduced to 15 wt. %, the resulting refractory (J in the Sutor patent) had only a 60 psi room temperature modulus of rupture. A room temperature modulus of rupture of less than about 100 psig is indicative of a refractory which possesses inadequate handling strength.
One important use of insulating alumina refractories which have low silica contents is as backup refractory liners for incinerators. Typically, incinerators are equipped with both a primary and a secondary liner brick. The primary liner brick is exposed to the internal temperatures of the incinerator. The secondary, or backup liner brick is interposed between the primary brick and the outer walls of the incinerator and acts to insulate the outer walls from the internal temperatures of the incinerator. The primary brick is commonly composed of a relatively non-porous brick which has a relatively high thermal conductivity. On the other hand, relatively porous refractories are especially well suited as backup liners, because, in general, more porous materials are poorer thermal conductors, and, thus, make better insulators.
The need, therefore, exists to find an alumina insulating refractory essentially free of silica, yet that can be dry pressed and fired into usable shapes.